South Korea's total fertility rate has hit a record low of 0.98 in 2018, a clear warning Asia's fourth-largest economy may face a population decline.

The 2018 figure is far below the replacement level of 2.1 that would keep South Korea's population stable at 51 million and also represents a sharp drop from the 4.53 of 1970, when the government began to compile data.

South Korea's crude birthrate - the number of births per 1000 people per year - also came to an all-time low of 6.4 in 2018.

"Total fertility rate of less than 1 is extremely low and means the population could decline at a very fast pace in the next 30 to 60 years," Kim Jin, director of Statistics Korea's vital statistics division, said.

South Korea is the only country among members of the Organisation for Economic Cooperation and Development where the total fertility rate fell below 1, Kim pointed out, though there's no OECD data on 2017 and 2018.

The average total fertility rate of the group of 36 mostly rich nations stood at 1.68 as of 2016, the latest year for which statistics are available.

The number of newborns in South Korea came to 326,900 in 2018, down 8.6 per cent, or 30,900, from 2017. Last year's figure also marked the lowest number of newborns since record keeping began.

The decline in childbirths is blamed on a sharp fall in the number of marriages and a growing number of women who are unmarried in recent years.

The number of people tying the knot hit a record low of 257,700 in 2018, marking the seventh-straight year of decline.

Some young South Koreans are opting to distance themselves from life's three major milestones - dating, marriage and having children - because they cannot find decent jobs amid a prolonged economic slowdown.

Kim said the portion of single women aged 30 to 34 jumped to 37.5 per cent in 2015, up from 10.7 per cent in 2000.